The technology of ultrasonic inspection has been developed since the World War II. In the beginning, it is used for the national defense and the military affairs. Until 1950s, the ultrasonic inspection technology started to be widely employed on the medical treatments. In the area of the ultrasonic inspection, ultrasonic transducer plays a very important role thus attracting the industry/government/academia to plunge into the research in the past decades, and the related technologies are also getting more and more mature now. Among all the ultrasonic transducers, the piezoelectric transducer was kept the main stream for a long time.
The so-called piezoelectric effect includes both of the direct piezoelectric effect and the converse piezoelectric effect. Under the direct piezoelectric effect, a piezoelectric body, when put into an electric field, will be elongated along the direction of the electric field according to the elongating of the electrical dipole moment thus transferring the mechanical energy into electric energy. On the contrary, under the converse piezoelectric effect, if the piezoelectric body was pressed, the electrical dipole moment thereof will be shortened. In order to resist this tendency, the piezoelectric body thus will induce voltage for trying to keep the original state. With such character, the piezoelectric transducer transfers the electrical signals into the sonic signals, and also can transfer the sonic signals into the electrical signals thus being able to regard as a probe in the ultrasonic inspection. The common material of the piezoelectric body can be the ceramic, such as BaTiO3 and PZT, and the single crystal materials, such as quartz, tourmaline, tantalates, and columbate. However, the piezoelectric transducer still exit some disadvantages, for example the cost of such piezoelectric transducer is too high, and the oscillation of the crystal lattice will easily debase the bandwidth and the sound pressure. Moreover, the difference between the impedances of the piezoelectric material and that of the air is so large as to cause the unmatched phenomenon thus resulting in large reflection of the sonic signals in the contact interface and diminish the inspection efficiency. In addition, for the limitation of the resolution and the bandwidth, the piezoelectric transducer is hardly to be used for the precise inspection in nano-level.
Instead of the piezoelectric transducer, the micro capacitive ultrasonic transducer has become the main stream of the ultrasonic transducer research. The related patents have also been gradually accumulated recently, such as U.S. Pat. Nos. 6,426,582, 6,004,832, and 6,295,247 and so on. Please refer to FIG. 1, which shows the basic structure of the micro capacitive ultrasonic transducer. A plurality of the support pedestals 12 is formed on the substrate 11, and the oscillation film 13 with an upper electrode 14 thereon is formed on the support pedestal 12. Wherein, the substrate 14 doped with impurities to get conductivity is used to be the lower electrode for forming a capacitance structure with the upper electrode 14. The oscillation cavity 15 composed of the substrate 11, the support pedestal 12, and the oscillation film 13 is used to provide the space for oscillation when the oscillation film 13 is vertically oscillating. Such micro capacitive ultrasonic transducer possesses the following merits: (1) larger bandwidth; (2) easily to form large density array; (3) simply to be integrated with the front-end circuits on the same wafer; and (4) being able to largely manufacture thus reducing the cost.
In fact, the important character of the micro capacitive ultrasonic transducer is the design of the oscillation cavity and the oscillation film, so the geometric parameters of the oscillation cavity and the oscillation film, such as the radius and the thickness of the oscillation film and the distance between the upper electrode and the lower electrode, are rigidly related to the efficiency of the ultrasonic transducer. Therefore, it is very important to control all of such geometric parameters more stably and more uniformly in the manufacturing process. Please refer to FIG. 2A to 2C, which are the schematic views showing the traditional method for manufacturing the micro capacitance ultrasonic transducer of the prior art. Firstly, a substrate 21 is provided, and then a support film 22, an oscillation film 23 and a conductive layer 24 are successively formed on the substrate 21. A plurality of holes 25 that penetrates the oscillation film 23 and the conductive layer 24 then is 1o generated after the procedure of photolithography and etching. Finally, through the plurality of holes 25, the support film 22 can be etched to form a plurality of oscillation cavities 221 thereon. Because of the character that the selectivity of the etching rate on the support film 22 and the oscillation film 23 is different, the etching solution that preferentially etches the support film 22 rather than the oscillation film 23 is used to form a plurality of oscillation cavities 221 thus completing the whole ultrasonic transducers. Wherein, the shape of the oscillation cavities 221 is approximate cylinder that expanded from the center of the holes 25. However, such method is hardly to control the precise shape of the oscillation cavities and cannot provide the check mechanism. It only depends on the experience so that many vibrations in the process, such as the variation of the etching solution concentration, will very easily cause the variation of the geometrical size of the oscillation cavities 221 further affecting the character of the whole transducers.
Moreover, the plurality of holes 25 used for the entries of the etching solution and the exits of the etching by-products will easily cause the contamination of the oscillation cavities 221, remaining certain residues on the wall of the cavities thus affecting the characters of the transducer. The present invention thus provides a new method not only for overcoming the aforesaid disadvantages but also for improving the character of the ultrasonic transducers.